The present invention resides generally in the field of techniques for quantifying analytes in liquid samples. More particularly, the invention relates to a process and an apparatus for treating a liquid sample to separate and concentrate an analyte, for example for introduction into a device for generating a signal relative to the concentration of the analyte.
As further background to the invention, membranes has long been studied as a sample interface for mass spectrometers. The first example of this type of technology was described by G. Hoch and B. Kok, Arch. of Biochem, and Biophys. 101 (1963) 171. Configuration changes in the membrane inlet design over time gradually increased the sensitivity of the technique with the most dramatic results being obtained through the use of the direct insertion membrane probe which positioned the membrane in the mass spectrometer source (M. Bier et al., Anal. Chem. 59 (1987) 597; R. G. Cooks et al., U.S. Pat. No. 4,791,292 (1989)). Membrane configuration where the membrane was located remote the mass spectrometer source remained problematic and was plagued by poor reproducibility and memory effects.
One of the most successful remote membrane designs was described by Slivon et al., Anal, Chem. 63 (1991) 1335. In this configuration the capillary silicone membrane was placed in a tubular chamber an the liquid sample flowed across the outside of the membrane. Analytes crossed the membrane by a process of pervaporation to the internal diameter where they drifted into the mass spectrometer source for analysis. Although reasonably good detection limits were obtainable, Slivon's design still suffered from some of the previous problems such as poor reproducibility.
Jet separator devices were originally designed as an interface between a gas chromatograph and a mass spectrometer. Early on in gas chromatography/mass spectrometry (GC/MS), packed chromatography columns were used. A typical packed column included a 1/8" diameter glass or stainless steal tube of variable length packed with a solid stationary phase. The gaseous sample passed through the column in a carrier stream which was typically hydrogen, helium or nitrogen. The problem of interfacing a mass spectrometer to a gas chromatograph was that the carrier gas stream volume was too high for the mass spectrometer to handle. A means of removing the excess carrier gas was required to provide an effective interface. Many devices were designed for this purpose, but the most successful was the jet separator. Generally, a jet separator includes a pair of needle jets separated by a small gap in an evacuated chamber. The heavier analyte molecules pass across the gap and continue into the mass spectrometer while the lighter carrier gas molecules that have less momentum are pumped away at the gap.
In light of the background in this area and the constant need to improve detection limits in analytical equipment such as mass spectrometers, there is a continued demand for improved processes and apparatuses for conditioning samples to concentrate analytes of interest for analysis. The present invention addresses this need.